In recent years, fuel cells have been focused as a power plant of electric vehicles because of its cleanness and excellent energy efficiency. In the fuel cell, oxygen is fed to the cathode and hydrogen is fed to the anode, and electricity is generated with the reaction between hydrogen and oxygen. For the purpose of supplying oxygen to the cathode, for example, a compressor is used to feed oxygen-containing air to the fuel cell.
Upon feeding oxygen-containing air to the fuel cell, the temperature of the air is raised by the compressive force of the compressor. The fuel cell efficiently generates electricity at a particular temperature range (80 to 90° C. in the case of polymer electrolyte fuel cell). However, air compressed by the compressor rises, for example, to 120° C. If air having such a high temperature is fed to the fuel cell, effective generation of electricity cannot be achieved. For this reason, air, before supplying to the fuel cell, passes through an intercooler and is cooled to a predetermined temperature, and thereafter the cooled air is fed to the fuel cell.
At the start of the fuel cell, the fuel cell is often cool, and effective generation of electricity is not achieved. Therefore, the fuel cell has to be heated (warmed up) instantly to a certain desired temperature at the actuation of the fuel cell. This is particularly important if the fuel cell is mounted on an electric vehicle.
In a conventional method, the fuel cell is heated by an electrical heater driven by a battery or commercially available power source or by a hydrogen-combustion heater such as disclosed in U.S. Pat. No. 6,103,410.
However, in the conventional method, since the electric heater or hydrogen-combustion heater is employed only for the purpose of warming up the fuel cell, consumption of electricity or hydrogen increases. And if the fuel cell is mounted on an automobile, running distance per unit fuel consumption decreases. In an electric heater employing a commercially available power source, there is a drawback in that electricity has to be introduced from the outside. Furthermore, the whole fuel cell system increases its size because there is a need to provide a dedicated electric heater or hydrogen-combustion heater.
If air temperature is low in a cold area or in the wintertime, the temperature of exhaust gas from the fuel cell is extremely low at the time of start up. In this instance, consumption of electricity or hydrogen will increase seriously for warming up the fuel cell. Furthermore, moisture in the fuel cell may be frozen below freezing temperature, which causes the fuel cell to generate little electricity. Therefore, it is necessary to warm up the fuel cell instantly.
In view of the above, a first object of the present invention is to provide a warm-up apparatus for a fuel cell, which enables quick warming-up of the fuel cell at the time of start up, and which does not require a dedicated electric heater or hydrogen-combustion heater.
Meanwhile, upon warming up the fuel cell, high-temperature air that is warmed by the compressor may be fed directly to the fuel cell. This allows the fuel cell to be warmed up instantly by the high-temperature air.
However, in a conventional method, high-temperature air is fed only to the cathode of the fuel cell. Therefore, the anode of the fuel cell is warmed up by heat that is transferred from the cathode through the membrane. For this reason, the anode is warmed up after the cathode, leading to a drawback that the whole fuel cell system takes time to complete a warm-up.
The anode of the fuel cell is also moisturized by an anode humidifier. Therefore, below freezing temperature, devices in the anode system are most likely frozen. However, since high-temperature air is fed from the compressor to the cathode of the fuel cell, warming-up is carried out merely for the cathode humidifier, and the anode circulation device and the anode humidifier are not warmed up at all.
In view of the above, a second object of the present invention is to provide a warm-up apparatus for a fuel cell, which enables warming-up of the fuel cell not only for devices in the cathode system but for devices in the anode system, and which enables warming-up for both the cathode and the anode to thereby warm up the whole fuel cell system quickly.